CS215 TCP Westwood Control Model Development and Stability Analysis Hu, Kunzhong Dong, Haibo Mentor: Wang, Ren Professor: Gerla, Mario March 21, 2001

Outline Introduction TCP Tahoe TCP Reno TCP Westwood TCP Bandwidth Estimation TCP Westwood Throughput Calculation Model TCPW model performance Stability and Fairness Analysis Conclusion and Future Work CS215

TCP Tahoe Slow Start Congestion Avoidance

CS215 TCP Reno Slow start Congestion Avoidance Fast Retransmit Fast Recovery Time Segments w thi w pi 1

Shortcomings of current TCP congestion control After a sporadic loss, the connection needs several RTTs to be restored to full capacity It is not possible to distinguish between packet loss caused by congestion (for which a window reduction is in order) and a packet loss caused by wireless interference The window size selected after a loss may NOT reflect the actual bandwidth available to the connection at the bottleneck CS215

TCP Westwood Estimation of available bandwidth (BWE): –performed by the source –computed from the arrival rate of ACKs, smoothed through exponential averaging Use BWE to set the congestion window and the Slow Start threshold

CS215 TCP Westwood (Cont.) When three duplicate ACKs are detected: –set ssthresh=BWE*rtt (instead of ssthresh=cwin/2 as in Reno) –if (cwin > ssthresh) set cwin=ssthresh When a TIMEOUT expires: –set ssthresh=BWE*rtt (instead of ssthresh=cwnd/2 as in Reno) and cwin=1

CS215 Link capacity Throughput Time CS215 Bandwidth Estimation RTT b1b1 b2b2 b k-1 bkbk b k+1 tktk t k-1 t k+1 time Expenential Increasement

Time Segments t k*RTT w thi w pi 1 i th cycle delay i CS215 TCPW Throughput Calculation Model

CS215 Model Performance Comparison of model predication with NS2 simulation for TCP Reno and TCP Westwood  = 45 Mbps,  = 70 ms, MSS = 1340 Byte

CS215 Model Performance (cont.) Comparison of window size evolution for TCP Reno and TCP Westwood  = 45 Mbps,  = 70 ms, MSS = 1340 Byte, error rate = 0.01%

CS215 SES1 SES2 DES2 DES1 SW1 SW2 BW B 11 22 CS215 TCPW Stability and Fairness Analysis Two Sources (SES1,SES2), Two Switches (SW1, SW2), Two Destinations (DES1, DES2) Bandwidth of the shared link is  [seg/sec] Propagation delay between SESi and DESi is  I Segment size is MSS bytes Buffer size of SW1 is B

CS215 X w 1 (t) w 2 (t) wewe wsws wsws wewe ( w 1 (t), w 2 (t) ) Segment Loss Line Efficiency Line Fairness Line Efficiency Area CS215 TCPW Stability and Fairness Analysis (Cont.) TCP Connection Window Size Graph Fairness Line, w 1 (t) = w 2 (t) Efficiency Line, link is fully utilized Segment Loss Line, below, no loss. w e = w 1 (t) + w 2 (t) w s = w 1 (t) + w 2 (t) + B

CS215 TCPW Stability and Fairness Analysis (Cont.) SES1 SES2 DES2 DES1 SW1 SW2 BW B 11 22 Window size of connection j when ith segment is lost Threshold window size for connection j in ith cycle. Time between i-1th and ith packet loss Throughput of connection j

Summary and Future Plan An effective throughput calculation model is developed. For any given packet loss rate, the evolution of TCP Westwood and TCP Reno window size and connection throughput are properly predicted using this model. Fairness and stability analysis shows that TCP Westwood is fair but lacks stability Further attention will be paid on the different methods of calculating the estimated bandwidth and on the throughput calculation model improvement. CS215